The incident coincided with a prolonged push by North Korea for talks to reduce military tensions that escalated sharply after Pyongyang conducted its fourth nuclear test in January.

The two North Korean vessels crossed the Yellow Sea border around 7:30am (2230 GMT Thursday) but swiftly retreated after a South Korean navy patrol boat fired five warning shots, a ministry official said.

The de-facto maritime boundary between the two Koreas - the Northern Limit Line - is not recognised by Pyongyang, which argues it was unilaterally drawn by US-led United Nations forces after the 1950-53 Korean War.